The present invention relates, in general, to integrated circuits and, more particularly, to digital-to-analog converters.
For a cellular voice system, the full band of channel frequencies received from an antenna is digitized at baseband or at a narrowband Intermediate Frequency (IF). Then the received channel frequencies are converted by a Digital Signal Processor (DSP) to voice band signals for transfer over local phone lines. In the transmit chain, DSPs reconvert the voice band signals to digitized multiple voice channels that are frequency multiplexed subcarriers. Digitization at baseband or at IF avoids many problems by filtering off-channel signals.
The digitized signals are typically converted back to an analog signal using Digital-to-Analog Converters (DACs). DACs can be either low precision or high precision depending on the number of input data signals they receive. A ten bit DAC is considered a lower precision DAC than a fourteen bit DAC. In other words, a higher precision DAC receives a greater number of input data signals and provides finer increments of current values in generating an output current than a lower precision DAC.
A high precision DAC could be implemented using a plurality of sufficiently accurate low precision DACs. For example, a high precision fourteen bit DAC could be implemented using sixteen ten bit DACs. The sum of the full-scale currents from the individual low precision DACs is the full-scale current of the high precision DAC. The full-scale currents from each of the low precision DACs are matched to a reference current to provide sufficient accuracy and to calibrate the high precision DAC.
Further, a high precision DAC can be implemented using a plurality of low precision DACs in conjunction with a spare low precision DAC. The spare low precision DAC maintains the current of the high precision DAC while one of the low precision DACs is calibrated. Thus, the spare low precision DAC allows periodic calibration of the high precision DAC without taking the high precision DAC off-line.
Calibration of the low precision DACs is initiated by replacing one of the low precision DACs with the spare DAC. The spare DAC maintains the current of the high precision DAC while the low precision DAC that has been replaced is calibrated to have a current that matches the reference current. Then, another low precision DAC is selected and calibrated. This process is repeated until the high precision DAC is calibrated. However, this technique of randomly selecting a DAC for calibration and replacing the DAC to be calibrated by the spare DAC still produces a high precision DAC having a nonlinear output signal.
Accordingly, it would be advantageous to have a method and a circuit for calibrating a DAC. It would be of further advantage to have the ability to calibrate the DAC while not taking the high precision DAC off-line or out of service.